1. Technical Field
The present invention relates to a hydrogen storage material, and more particularly, to a hydrogen storage electrode comprising the active hydrogen storage material.
2. Background Art
The study of rare-earth hydrogen storage alloys began with LaNi.sub.5. The crystal lattice of LaNi.sub.5 expands dramatically during the hydrogen charge-discharge process, thereby resulting in a loss of the alloy's capacity for storing hydrogen. Over the past twenty years, efforts have been made to find suitable active materials for hydrogen storage electrodes. The ideal material for hydrogen storage electrodes should have (1) an effective electrochemical capacity, (2) an appropriate equilibrium decomposition pressure of hydrogen, (3) a prolonged life of charge-discharge, (4) corrosion resistance, (5) excellent electrocatalysis and (6) inexpensive raw materials.
In order to obtain a suitable hydrogen storage material, efforts have been made to improve the composite structure of hydrogen storage alloy electrodes, as by adding Mn to the conventional LaNi.sub.5 alloy to reduce the plateau pressure, adding Al to increase its corrosion resistance, and adding Co to reduce the expansion of the crystal lattice. The life of hydrogen storage alloy electrodes is prolonged by the addition of Mn, Al and Co. The charge-discharge life-cycle has been increased by such additions from less than twenty cycles to about a hundred cycles. (JP01231268; J Materials Science, 1983, 18, 321-24; Progress in Batteries & Solar Cells, 1989, 8). However, to date, materials for hydrogen storage electrodes have not been reported in any literature with which a secondary battery has met the IEC standard with a life of up to 500 cycles. Although the addition of Co prolongs the life of the electrode, it falls far short of reaching the IEC standard. Moreover, Co is a very expensive raw material.